This invention relates to a novel mercury emitting structure with a holder having an alloy including an intermetallic compound consisting mainly of yttrium, nickel and mercury.
Where mercury is filled in a tube such as a nixi tube and low-pressure mercury vapor discharge tube, a predetermined amount of mercury is supplied from a "mercurydoser" through supply and exhaust tubes into the tube. In the above-mentioned method, however, it is difficult to accurately seal a required amount of mercury, for example, several milligrams to tens of milligrams of mercury, into the tube, and a sealed amount of mercury is deviated, an amount of one in few parts to several-fold, from the required amount. Furthermore, a mercury consumption, as well as a raise in cost due to a mercury recovery etc., is involved and an environmental pollution at a workshop due to evaporation of mercury is also involved. These disadvantages are due mainly to the fact that mercury is treated in the form of liquid.
In order to avoid such a situation several attempts have been made to seal mercury in the form of solids into a tube. One method is to seal an alloy (i.e., amalgam) of mercury with indium, cadmium and the other metal into a tube so as to mainly regulate a mercury vapor pressure in the tube. This method is suitable for a low pressure mercury vapor discharge tube as used under a high temperature circumstance. However, this method is not employed in a tube as used in an normal circumstance, since an intra-tube mercury vapor pressure is too low. A second method is to seal a predetermined amount of inorganic compound of mercury into a tube and thermally decompose, after evacuation and sealing steps, the inorganic compound from outside of the tube (by for example, a high frequency wave heating) to liberate a predetermined amount of mercury in the tube while an unrequired gas -- including, for example, oxygen, sulfur compound etc. -- simultaneously emitted from the inorganic compound is reacted with a suitable metal such as titanium, zirconium, iron etc. This method is disclosed in U.S. Pat. Nos. 1,855,901 (mercury sulfide and iron are used), U.S. Pat. No. 3,230,027 and U.S. Pat. No. 3,385,644 (mercury oxide and reducing metal are used) and U.S. Pat. No. 3,401,296 (mercury pyrophosphate and reducing metal are used). These disclosed methods present difficulty from a practical viewpoint, since it is impossible to completely prevent emission of an obnoxious gas during a heating time. The third method is disclosed in U.S. Pat. Nos. 3,318,649. U.S. Pat. No. 3,318,649 discloses the use of an alloy formed from mercury and a metal selected from groups IA, IIA or III including the lanthanides and actinides of the periodic table. Although the rare earth metals are disclosed in this patent, no detailed example is disclosed therein. This patent also discloses the concept of making an alloy of magnesium and mercury stabilized by the use of nickel. Where the above-mentioned alloy is sealed in a tube and heated by a high frequency wave, magnesium is evaporated at a temperature of 600.degree. to 700.degree. C due to its high vapor pressure and, in consequence, it is not turned to advantage from the practical viewpoint. The fourth method is disclosed in an article of "Journal of Metals" by P. Pietrokowsky (February 1954). This article discloses a mercury emitting getter in a holder in which an intermetallic compound of titanium, zirconium and mercury -- for example, a thermally stable metal compound such as TiHg, ZrHg, .gamma.-Ti.sub.3 Hg, .delta.-Ti.sub.3 Hg etc. -- is received. Such a mercury emitting getter is disclosed in U.S. Pat. Nos. 3,733,194, U.S. Pat. No. 3,772,976 and U.S. Pat. No. 3,657,589 and Japanese Patent No. 5659/1974. Recently, such mercury emitting getter is commercially available under the name of "GEMEDIS" (S.A.E.S. Getters S.pA., Italy). This getter is a Nickel-plated iron ribbon with Ti.sub.3 Hg compression-bonded on one surface side thereof and Zr-Al based alloy compression-bonded on the other surface side thereof. The "GEMEDIS" has a disadvantage that at a lower temperature a mercury emitting speed is slow due to the presence of the mercury emitting getter and at a higher temperature a metal, constituting a ribbon-like holder for holding the mercury emitting getter, is easily evaporated.
A technical report TR-22, p5 (FIG. 5), issued by S.A.E.S. Getters S.p.A., Italy, shows that in order to effect a 90% mercury emission the mercury emitting getter should be heated for 10 minutes at a temperature of 900.degree. C under a vacuum of 10.sup.-4 mmHg. However, such getter can not be applied to a high-speed tube manufacturing equipment in which a tube is manufactured at the rate of one per a few seconds. The technical report further shows on page 5 that, when the mercury emitting getter is heated for 15 to 30 seconds at a temperature of 900.degree. C .+-.100.degree. C, a 70 to 80% mercury emission can be obtained. Since the "GEMEDIS" has an intermetallic compound of mercury held by the nickel-plated iron ribbon-like holder, if the holder is too heated, nickel is evaporated and deposited on a phosphor-coated tube wall and cathode, resulting in lowered tube characteristics. If the evaporated nickel is deposited onto the cathode, a cathode emission characteristic is degraded with the resultant raised starting voltage. If, on the other hand, the evaporated nickel is deposited onto the tube wall, smears are formed on the phosphor-coated tube wall with the result that a light emitting characteristic, brightness characteristic etc. are lowered. Furthermore, it is difficult, from the standpoint of a tube manufacture, to severely regulate the mounting position of the holder to a mount. When the holder is heated by a high frequency wave from outside of the tube, difficulty is encountered in effecting the uniform heating of the holder and the holder is heated in varying temperature dependent upon the mounting position. If, therefore, the holder is not set to a temperature below 900.degree. C, it is overheated and nickel tends to be evaporated. Suppose that the mercury getter is applied to a low-pressure mercury vapor discharge tube. Since in this case an inert gas such as an argon gas is sealed under a vapor pressure of about 2 to 3 mmHg into the tube, the mercury vapor pressure is slower than that disclosed in the above-mentioned technical report. Even under good conditions it is actually difficult to obtain a more than 75% mercury emission. In consequence, even when the "GEMEDIS" is used, a total amount of mercury sealed in a tube is decreased only to a lesser degree.